Place tea bags and mint in a heat proof pitcher. Add boiling water. Steep at least 10 minutes. Strain into another pitcher and discard mint and tea bags. Stir in sugar. Put blackberries in a blender or food processor; strain through a fine sieve. Discard pulp and seeds. Stir blackberries into tea. Taste and adjust sugar as desired. Chill. Serve over ice garnished with mint leaf and 2 or 3 blackberries.